The present invention relates to manufacturing gas turbine engine components and, more particularly, to drilling a cooling hole with a diffuser opening through the surface of a gas turbine engine component using only a laser beam in a single laser drilling operation.
Gas turbine components, such as blades, vanes and the like, operating in high temperature gas are often hollow with an air coolant passageway therethrough. openings through the blade from the inner coolant passage permit the coolant air to flow out thereby permitting a coolant flow passing coolant air through the material of the component itself to cool it and also providing a film cooling of the outer surface of the component. A diffuser shape of the opening lowers the terminal velocity thereby increasing the effectiveness of the film cooling of the component surface.
To facilitate the distribution of the coolant air substantially completely over the convex and concave surfaces of the blade airfoil or platform, the hole openings are preferably shaped like a trapezoid with the downstream portion of each hole opening, at the airfoil surface, flaring or widening relative to the narrower upstream portion of each hole opening. The increasing cross-sectional area of the hole opening functions as a diffuser to reduce the velocity of the cooling airstreams exiting the holes; the lower velocity airstreams are more inclined to cling to the blade surface for improved cooling rather than separate from the blade.
One method and apparatus for forming a shaped cooling hole in an airfoil, similar to that described above, is disclosed in U.S. Pat. No. 4,808,785 to Vertz et al. Vertz discloses basically a two step process using Electric Discharge Machining (EDM) and laser drilling; an EDM step is performed to create the diffuser-shaped hole opening on the airfoil surface and a laser drilling step is performed to penetrate through the airfoil and into the hollow interior of the blade. The entire process is independent of which step is performed first. This two step process typically takes longer than if the hole with a diffuser-shaped opening could be formed in substantially a single hole drilling or forming operation.
Another method for forming a shaped cooling hole is disclosed in U.S. Pat. No. 4,737,613 to Frye. Frye discloses laser machining a cooling hole in a blade by laser firing to drill through the workpiece at a selected location and with firing continuing moving the laser to other selected locations, with an hourglass shape being machined. While this method is effective in forming a cooling hole, the hourglass shape is not desirable for many applications.
In order to create an effective distribution of cooling air through the diffuser opening while maintaining the strength and integrity of the component wall through which the cooling hole is being drilled, it is desirable that the cooling hole have a metering hole which has a circular cross section of relatively constant diameter leading to a diffuser which has a trapezoidal cross section opening at the surface. It is desirable that gouging or other distortions of the circular cross section shape of the metering hole and/or contamination of the component by debris entering the hollow component and/or back wall strikes by the laser be avoided.
During the laser drilling of cooling holes the hollow gas passages of the component are generally filled with a backer material to absorb or dissipate the excess laser beam energy that exits the cooling hole thus preventing back wall strikes. However, in many instances the amount of backer material is limited as the hollow gas passages of the component are small and narrow. If excess laser energy is permitted to exit the metering hole the backer material can be consumed and the internal back wall of the component (ie. the internal surface facing the exit of the metering hole) can be damaged. Prior art processes which laser drill the metering hole first and then laser drill the diffuser shape result in an excessive amount of laser beam energy exiting the metering hole and can result, not only in gouging and damage to the metering hole shape, but debris entering the component and back wall strikes.
It is accordingly an object of the present inventor to provide a method for laser drilling a cooling hole with a circular cross sectional metering hole and a diffuser opening.
It is another object of this invention to provide a method of laser drilling in a single operation a cooling hole with a diffuser opening which provides a metering hole without gouging or contamination.
Briefly, the present invention provides a method of laser machining a cooling hole in a hollow gas turbine component, the cooling hole including a circular cross section metering hole which extends from the inner surface of the component and a diffuser with an opening which extends from a shape locus in the metering hole to a trapezoidal cross section opening at the outer surface of the component by firing a series of laser pulses at an acute angle at the outer surface of the component toward the shape locus to create the diffuser opening, with the laser pulses penetrating the component outer surface up to but not substantially beyond the shape locus, then firing the laser through the shape locus and the inner surface of the component to create the metering hole having a circular cross section. In an alternate embodiment the initial laser pulses are fired to create the central portion of the diffuser opening, then firing the laser to create the metering hole, followed by firing the laser along a peripheral portion of the diffuser opening to create the diffuser opening.